A circle has a radius of $9$. An arc in this circle has a central angle of $80^\circ$. What is the length of the arc? {18\pi} {80^\circ} \color{#DF0030}{4\pi} {9}
Explanation: First, calculate the circumference of the circle. $c = 2\pi r = 2\pi (9) = 18\pi$ The ratio between the arc's central angle $\theta$ and $360^\circ$ is equal to the ratio between the arc length $s$ and the circle's circumference $c$ $\dfrac{\theta}{360^\circ} = \dfrac{s}{c}$ $\dfrac{80^\circ}{360^\circ} = \dfrac{s}{18\pi}$ $\dfrac{2}{9} = \dfrac{s}{18\pi}$ $\dfrac{2}{9} \times 18\pi = s$ $4\pi = s$